Overview of American Deserts

(Left) A view of the Sonoran Desert in the Lower Colorado River Valley.
(Right) Giant Saguaro Cacti in the Sonoran Desert.

While flying in a commercial jet over the American West recently, I was fascinated by the desert landscape thousands of feet below. There were vast open stretches of land and mountains arising out of the desert floor all around. There were interesting formations in hillsides across the expansive desert that grabbed my attention during the flight. Although the landscape was interesting, I was glad I was flying over this barren, uninhabited territory, instead of driving across it in a car. These deserts are the product of multiple climatic processes, the most influential being one that begins at the equator, thousands of miles and many countries away.

Deserts cover a large portion of the American West, from California to central Texas, from Northern Mexico to Southern Idaho. There are four deserts that comprise this region, the Great Basin, the Mojave, the Sonoran, and the Chihuahuan. Like the other major deserts around the world, the climate of these North American deserts is influenced by atmospheric circulations that start at the equator. The equatorial region receives more radiation than any other area on earth. This creates warm air that is less dense, causing it to rise through the atmosphere, and travel either north or south. This air mass finally descends at the subtropical zones in the Northern and Southern Hemispheres. These circulation patterns are part of the phenomenon known as Hadley Cells. As the air descends in the sub-tropical zone, it is compressed and heated, creating areas of high air pressure. Warm air can absorb more water vapor than cooler air, so the descending air becomes dry. This warm dry air defines the North American desert climate. A further explanation of Hadley Cells can be found at www.earlham.edu/~biol/desert/whatis.htm

Another atmospheric process that influences desert climates is the rain shadow effect on the lee side of mountain ranges. When air rises as it hits the windward side of a mountain slope, it cools due to the concept that the temperature decreases as the altitude increases in the lower atmosphere. As the air cools it reaches its dew point, the point at which air can hold no more moisture, causing water vapor to condense into clouds and precipitation. As the air moves down the other side of the mountain, the lee side, any moisture that is still left in the air is evaporated as it warms up through compression as it descends. The air reaches the base of the leeward side warm and dry. This effect is notable east of the Sierra Nevada Mountain ranges and also influences the climate of the North American Desert in many locations. Much of the North American Deserts are also separated from the moderating effects of the ocean, leading to extreme temperatures. The interaction of these three climatic processes creates the landscape that I observed during my flight.